1. Field of the Invention
The present invention relates to a recording and/or reproducing apparatus for writing data into and/or reading data out of a recording medium.
2. Description of the Prior Art
Various apparatuses to write data into or read data from recording media are now available, including magnetic hard disc apparatuses, magnetic floppy disc apparatuses, optical disc apparatuses, optical card apparatuses and so on. With these apparatuses, a magnetic head or an optical head is used to write and/or read out data into/from a track defined on the surface of a recording medium.
These apparatuses have been widely used as information and video apparatuses. Recently, there is an increasingly strong demand for increasing the data storage capacity and making such apparatuses and recording media more compact in size. Thus it is essential to increase the data-packing density. For instance, magnetic disc apparatuses generally use discs of the so-called double-density type, each having a data packing capacity of two megabytes, or discs having even higher data-packing densities of ten or more megabytes. In order to use discs having a super high data-packing density of between five and ten megabytes, motors which are capable of a super high degree of accuracy are needed, so that instead of the conventional DC motors, linear pulse motors have been now devised and demonstrated.
However, the assembly of a disc apparatus with such a linear pulse motor requires an extremely high degree of accuracy, which increases the production costs. Therefore, instead of a linear pulse motor, a voice coil motor capable of positioning a magnetic or an optical read/write head properly on a desired track at a high speed has been used as the head driving means because they are available at reasonable prices.
When a voice coil motor is used as the means for moving a write/read head, in order to control the relative position of the head with respect to the disc, a linear encoder which is driven by the voice coil motor is employed so as to detect the displacement of a carriage upon which the head is mounted.
Meanwhile, in order to increase the data-packing density of the discs as described above, it is essential to reduce the width of each track and to reduce the pitch between the adjacent tracks, so that the number of tracks defined on one surface of the recording medium can be increased. However, when the track width and pitch are reduced, an impact exerted from the exterior of the recording and/or reproducing device can easily displace the magnetic head from the desired track, and in some cases, the head may be moved to the adjacent track. If such erroneous positioning of the head occurs when data is being written on the surface of the recording medium, the desired data cannot be read out from the desired track and the data stored in the adjacent track is destroyed.
The above and other problems encountered in the conventional data recording and/or reproducing apparatuses will be described in detail in conjunction with a magnetic disc apparatus utilizing a tracking servo system. The tracking servo system has been devised and demonstrated to alleviate the problem that the desired data sometimes cannot be correctly written into and/or read from a high density recording medium with many tracks by a head positioning apparatus of the type which moves the head only by a predetermined number of steps. The tracking servo system is such that servo data is read from the surface of the recording medium and the head is positioned on the desired one of a large number of data tracks defined at a high degree of density.
FIG. 1 is a block diagram illustrating the servo system of a conventional magnetic apparatus and FIG. 2 is a schematic view illustrating the tracks and servo data defined on a magnetic disc used in the apparatus shown in FIG. 1.
Referring now to FIGS. 1 and 2, reference numeral 24 represents a magnetic disc spun by a spindle motor (not shown); 26 represents a magnetic read/write head for reading data out and writing data into the surface of the magnetic disc 24; 1003 represents a write circuit for applying a write current to the magnetic head 26; 1004 represents a read amplifier for amplifying the data read out from the surface of the magnetic disc 24 by the magnetic head 26; 1005 represents a servo data signal detector for detecting the servo data signal from the amplified read out data from the amplifier 1004; 1006 represents a positioning error detector for generating a positioning error signal in response to the servo data signal detected by the detector 1004; 1007 represents microcomputer responsive to the output signal from the detector 1006 for computing motor control data; 1008 represents a D/A converter for converting the motor control signal derived from the computer 1007 into an analog motor control signal; 1009 represents a power amplifier for amplifying the analog motor control current signal derived from the D/A converter 1008; and 1010 represents a linear motor for moving the magnetic head 26 in the directions indicated by the double arrow and perpendicular to the tracks defined on the surface of the disc 24, thereby positioning the head 26 over a desired track properly. A large number of tracks 1011 are defined over the surface of the magnetic disc 24 and each track 1011 is divided into a plurality of sectors. A servo signal region 1012 is defined between the adjacent sectors of each track 1011. Various kinds of servo signals may be used and in this specification, as an example, burst signals 1013 and 1014 having different frequencies f.sub.1 and f.sub.2 which are spaced apart from each other in the radial direction of the magnetic disc 24 and which are offset from the tracks by one half of the track pitch as shown in FIG. 2 will be described.
In the case of tracking along the center line of the track 01 (T01), the head 26 reads out both the burst signals 1013 and 1014 simultaneously at each servo region 1012 so that signals f.sub.1 and f.sub.2 having the same amplitude can be obtained by frequency discrimination. However, if the magnetic head 26 deviates slightly from the center line of the track 01 (T01) toward the adjacent track 00 (T00), the amplitude of the signal f.sub.2 becomes greater than that of the signal f.sub.1. In response to the difference in amplitude between the signals f.sub.1 and f.sub.2, a positioning error is computed and utilized to control the position of the magnetic head 26 by the microcomputer 1007.
In the conventional apparatuses of the type described above, servo data are intermittently obtained from the servo data regions. When an external impact is exerted on the apparatus, the magnetic head may deviate radially inwardly or outwardly from the center line of the predetermined track. But the microcomputer cannot detect the deviation when the magnetic head is away from the servo signal regions, so that the operation of the magnetic head is continued. If the deviation of the magnetic head from the center line of a predetermined track occurs during a writing operation, the recorded data recorded on the adjacent track are completely destroyed in the worst case.